PSpice Application Notes

APPLICATION NOTE 4 Other Common Components Resistors and capacitors are only part of the problem in making accurate PSpice simulations. Conductors (PCB traces and wires) and inductors also deviate from ideal as the frequency increases. Conductors A conductor that looks like a small resistor at DC has an increasing impedance with a frequency that is dependent on the physical dimensions of the conductor. Its inductance can be approximated by an inductor of about 20 nH per inch of length in series with the DC resistance. Thus, a conductor looks inductive at frequencies as low as 10 kHz up to the length that is about a quarter-wavelength long. At longer lengths, the conductor undergoes multiple pole and zero resonances like an antenna. The frequency where a conductor stops looking inductive and starts to act like an antenna can be found using the formula, F=2850/L where L is a quarter of the wavelength in inches, and F is in MHz. Thus, a conductor that is 10 inches long will behave like an antenna when the frequency is 285 MHz or greater. Most PCB traces are not long enough to act as antennas, but ribbon cables can be. On controlled impedance PCB's, the traces look like transmission lines. Even power and ground planes used in a PCB design don't escape frequency effects. The impedance of a ground plane doesn't look inductive at higher frequencies; it looks lossy. At higher frequencies, the skin effect of the plane starts to dominate and increase the planes impedance. The skin effect is proportional to the square root of the frequency, so it doesn't rise as fast as does that of a wire that is behaving inductively. Inductors Inductors vary greatly in shape and size depending on the exact job that they are to perform. Power inductors, like the type used in switching power supply output filters, are usually large structures that may self-resonate at frequencies from 500 kHz to 75 MHz. These power inductors are sometimes designed for low loss so they may have a large Q at resonance. The high Q gives rise to a rather narrow, sharp resonance. Above the resonance frequency, the inductor's shunt capacitance dominates. The shunt capacitance is usually large for a big power inductor because of the capacitive coupling among the many turns used. When modeling power inductors, the resonant frequency is based on the size of the core. Generally, the larger the core, the lower the self-resonant frequency. Ferrite beads used for EMI control are at the other end of the spectrum. Beads are designed for lossy operation and have very low Q values with relatively low inductance. The self-resonance peak is low and very broad, extending for several octaves of frequency. Beads are best modeled as an inductor with a small shunt resistance on the order of 50 to 100 ohms and a low shunt capacitance of 1-5 pF or less.

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